1. Field of the Invention
The present invention relates to a frequency detector for detecting the pressence of a predetermined frequency component in an electrical signal. The invention is especially suited for discriminating between single longitudinal mode (SLM) and multi-longitudinal mode (MLM) operation of a pulsed laser.
2. Description of the Related Art
Pulsed lasers generate coherent light beams by optical resonance in a suitable material. An output beam may be produced when the amplitude of optical oscillation in the material exceeds the losses therein. Although oscillation in a single longitudinal mode is desirable in most laser applications, relatively elaborate steps must be taken in the design of a particular laser to suppress oscillation in longitudinal modes other than the desired one. Oscillation in each longitudinal mode occurs at a frequency which is different from oscillation in the other modes.
A discussion of multiple longitudinal modes of oscillation in a laser cavity, as well as methods proposed to achieve single longitudinal mode operation, is found in a textbook entitled "An Introduction to Lasers and Their Operation", by D. O'Shea et al, Addision-Wesley Publishing Company, Chapt. 5, pp. 107-124 (1978).
Lasers which operate consistently in SLM are expensive to produce on a commercial production basis. Even intricate, well designed lasers adapted to suppress MLM operation generate a certain proportion of multi-mode pulses due to thermal noise and unpredictable changes in ambient conditions.
Pulsed SLM lasers are used extensively for scientific research in numerous technical fields. Often, a material will be irradiated with one or more laser pulses to observe and measure the effects of the radiation on the material. Various materials are affected differently by SLM and MLM pulses. In order to correctly identify and quantify the irradiation effects, it is necessary to discriminate between the results produced by SLM pulses and occasional MLM pulses. This has been accomplished in the past by connecting a high speed oscilloscope to the output of a photodetector which is irradiated by a portion of the laser beam. The temporal shape or profile of an SLM pulse is smooth, whereas one or more beat frequency components are superposed on the profile of an MLM pulse. Each beat frequency component is produced by interaction of two longitudinal oscillation mode frequencies at the difference therebetween. The researcher is able to visually distinguish MLM pulses from SLM pulses by their different shapes. However, this can be quite awkward, especially where the researcher is attempting to observe the effects of the pulses on the material being investigated in addition to the shape of the pulse on the oscilloscope, and is practically impossible at a pulse rate of more than two or three pulses per second. Applications requiring high power laser beams generally employ amplifiers for increasing the power level of a beam generated by a relatively small source laser. These amplifiers often include elements which are quite large and expensive, and are operated just below their material damage threshold for maximum efficiency. Whereas the damage threshold is not exceeded by SLM pulses, the interactive combination of frequency components in an MLM pulse can result in a composite pulse amplitude which exceeds the threshold. The result is catastrophic failure of the amplifier element.